A.) Industrial Heat Processes: PA0 B.) Ultrasound Waves: PA0 C) Optical Interferometers:
Industrial heat processes are controlled by measuring the temperature of the object or the workpiece which is either heated or cooled. The traditional method of determining workpiece temperature in an industrial furnace is to measure the surface temperature of the workpiece with a radiation pyrometer, or a thermocouple positioned near the workpiece or a contact thermocouple. There are limitations with both devices. Contact thermocouples have limited use because they scratch the workpiece surface when used in a continuous process i.e. strip lines, or they require drilling of a hole in the workpiece if used in a batch furnace or, in other applications they cannot be used. Radiation pyrometers are traditionally used for non-contact surface temperature measurements but suffer from inherent inaccuracies because of interferences by radiation from furnace hot walls and gases in the furnace. Further, the accuracy of radiation pyrometers is adversely affected by varying emissivity of the workpiece during thermal processing. The emissivity encountered in certain strip applications i.e. galvanizing, aluminizing, galvannealing, etc. preclude pyrometer direct applications in the sense that present schemes utilize a pyrometer and one of two emissivities. If one is wrong, the other is assumed correct. In those instances when contact thermocouples or pyrometers cannot be employed, secondary measurements are obtained and correlated to the expected temperature of the workpiece to control the process. For example, the temperature of the furnace gas is sampled and by means of empirical equations the process is controlled. In plasma arc applications or induction heating applications the electrical power inputted to the workpiece is controlled. In addition other measurements are taken and the process is controlled by the combination of measurements. For example, the furnace gas is sampled and its makeup analyzed to determine the extent that the heat treating process has progressed. Again, all of these controls are secondary in that something other than the workpiece is measured and from that measurement an expected characteristic of the workpiece is extracted.
With respect to measurement of temperature, traditional, nondestructive temperature measuring instruments lack any capability to measure temperature gradients within the workpiece. There are thermal processes where temperature uniformity (plus or minus 5.degree. F.) from surface to the core of the workpiece is critical to achieve uniform phase transformation reactions in the bulk of the workpiece. Traditional processing techniques provide a predetermined hold or soak time at which uniformity is to be achieved and this results in increased process cycle time etc.
With respect to measuring physical or chemical properties of the workpiece, there are no instruments which can nondestructively, directly measure the properties of the workpiece during the heat treat process, although theoretically there are, of course, ways to actually measure temperature gradients. As indicated above, secondary measurements are obtained and correlated to what the expected properties of the material would be. For example, in the heat treat carburizing process, the furnace or process gas is sampled to determine its carbon content and based on the measured variation in carbon content of the furnace gas the process is controlled on the underlying assumption that the carbon disassociated from the furnace atmosphere is uniformly infused into the workpiece case.
As noted in the NTIS report a significant body of information has been published on the generation of elastic waves in solids. It is known that when transient changes in the structure of a solid occur, elastic waves are generated on the surface and in the bulk of the workpiece. It is known that there are four types of waves which can propagate in solids, namely longitudinal and/or shear, Rayleigh or surface and Lamb waves. The longitudinal and/or shear waves travel through the bulk of the solid with the longitudinal waves being almost twice as fast as the shear waves. The Rayleigh waves travel only on the surface of the solid with speeds slightly less than the shear waves. The Lamb waves propagate only through very thin plates and have been used to measure the thickness of these plates. Longitudinal bulk waves and shear bulk waves have also been extensively used for detection of flaws, measurement of elastic properties of solids and monitoring of steel solidification.
It is known to use lasers to generate ultrasound waves on the surface of the workpiece. See for example Kaule U.S. Pat. No. 4,144,767. Further, it is known in the literature search of the NTIS report that three types of waves (longitudinal, shear, and Rayleigh) can be produced by laser in an unheated environment in aluminum, brass and various types of steel without any surface damage.
All of the references uncovered in the NTIS literature search used transducers i.e. conventional piezoelectric transducers to detect the sound wave. Clearly, placing the transducers in a heated environment either destroys the transducer or, at the very least requires extensive correcting circuitry to compensate for the temperature effect on the piezoelectric device which in turn can adversely affect the readings from the device.
As discussed in the literature search of the NTIS report, the industrial use of an optical interferometer to detect the movement of ultrasound waves at elevated temperature has not been uncovered. This is not surprising when it is considered that normal applications for interferometers require precise optical path lengths be established for the reference and signal light beams which cannot exist in an industrial setting. Accordingly, considering only variations in path beam length arising in industrial applications, one would not expect an interferometer to have the sensitivity to consistently measure ultrasound wave movement in an industrial setting with typical interferometer such as Twyman-Green, Michelson, Mach-Zehnder, Fabry-Perot. At the same time within the optical interferometer art, phase-shifting interferometers are well known and it is a characteristic of phase-shifting interferometers that the optical beam paths need only be set equal to one another within the coherence length of the beam light. However, phase-shifting interferometers require several phase-shifts in the reference beam to obtain the measurement i.e. see for example Gallagher U.S. Pat. No. 3,694,088, and the time required to generate multiple phase-shift readings, until the present invention, would prevent the use of phase-shifting interferometers to measure the surface movement of an object in response to an ultrasonic wave induced therein.